1. Field of the Invention
This invention relates to a power supply apparatus for supplying power to a plurality of loads by using a plurality of batteries as a power source, and an electronic appliance using the power supply apparatus.
Portable electronic appliances such as notebook-type personal computers generally use rechargeable batteries such as NiCad batteries, NiMH batteries, Li+ (lithium ion) batteries, etc., as power supplies, and charge the rechargeable batteries through charging circuits when AC adaptors are connected thereto.
A requirement for longer operating times of these portable electronic appliances, by increasing the battery capacities, has become important. This requirement can be satisfied, in principle, by connecting the batteries in series and then connecting the series circuits of the batteries in parallel, within the range of the withstand voltage of a power supply system. When a charging operation is conducted on the batteries connected in parallel, however, charging currents become different between the batteries in some cases due to the differences in impedance and differences in the charge state. Accordingly, there is the danger that a current exceeding a rated value is applied to some batteries.
When a discharging operation is made from the batteries connected in parallel to a load, discharge is made from one battery to another due to the differences in impedance or in charge state of the batteries, so that a current exceeding a rated value is passed in some cases.
For these reasons, it has been customary to employ a circuit construction in portable electronic appliances such that batteries connected in series are handled as a battery pack and the pack is individually charged and discharged. In portable electronic appliances employing such a circuit construction, a flexible construction must be used for the power supply, too, in order to cope with the diversie needs of the units constituting the appliances.
2. Description of the Related Art
A power supply apparatus using two batteries will be explained as an example of the prior art apparatuses. In the power supply apparatuses according to the prior art, loads inside the electronic appliance are dispersedly connected to a plurality of DC--DC convertors, and the input sides of all the DC--DC convertors are connected in common.
A first battery is connected to a first discharge switch, and this junction is connected to a second battery through a second discharge switch. A charging circuit is connected to the first battery through a first charge switch, and the charge circuit is connected to the second battery through a second charge switch.
The conventional portable electronic appliance uses either one of these two batteries as a power supply. When the first battery is used, the first discharge switch is turned ON so as to connect the first battery to all the DC--DC convertors and the second discharge switch is turned OFF so as to cut off the connection between the second battery and all the DC--DC convertors. When the second battery is used, on the other hand, the second discharge switch is turned ON so as to connect the second battery to all the DC--DC convertors and the first discharge switch is turned OFF so as to cut off the connection between the first battery and all the DC--DC convertors.
When the AC adaptor is connected, either one of the two batteries is the object of the charging operation. In other words, when the first battery is charged, the first charge switch is turned ON so as to connect the first battery to the charging circuit and the second charge switch is turned OFF so as to cut off the connection between the second battery and the charging circuit. When the second battery is charged, on the other hand, the second charge switch is turned ON so as to connect the second battery to the charging circuit and the first charge switch is turned OFF so as to cut off the connection between the first battery and the charging circuit.
In practice, this processing is executed by applying priority to the batteries. For example, when discharge of the first battery having higher priority is started and this battery becomes dead or empty, discharge is switched to the second battery while the charging operation of the first battery having higher priority is started. When the first battery is fully charged, the second battery is then charged.
As described above, the portable electronic appliance according to the prior art employs the construction wherein a plurality of DC--DC convertors of the appliance are handled as an integral unit, and the batteries collectively supply a voltage to them.
Accordingly, there arises the problem that when a unit requiring a large load current such as a wireless unit, etc., must be added to the appliance when the portable electronic appliance is upgraded, for example, the current exceeds the maximum supply current of the batteries and necessary power cannot be supplied.
Further, the conventional portable electronic appliance employs the construction wherein, when the batteries used as a power supply become dead or empty, they are merely switched to other batteries.
This results in the problem that when the load current of the DC--DC convertor becomes great for some reason or other, the apparatus cannot cope with such a situation. Further, the power supply apparatus does not at all consider how to extend the discharge life of the batteries.